1. Field of the Invention
The present invention relates to a bucket elevator-type continuous unloader.
2. Description of the Prior Art
A crane which is used at a wharf to unload coal, mineral ores, and other types of bulk cargo with a high efficiency is generally called an unloader. Such an unloader grabs and discharges the bulk cargo in the hold by means of a grab bucket. However, in order to further enhance the efficiency, a bucket elevator has come to be used in place of the grab bucket.
Such a bucket elevator-type continuous unloader has had problems in that, at the time of unloading the bulk cargo stowed in a ship's bottom, i.e., clearing goods from the bottom, there is a risk of damaging the bottom or the bucket as a result of a collision between the vertically moving bottom and a scraping section of the bucket elevator, and in that an uncleared portion remains in the bottom cleaning operation.
For this reason, a proposal has been made to transmit an electrical signal as the lower end of the bucket elevator reaches a certain distance from the ship's bottom (see Japanese Utility Model Examined Publication No. 54-36465). Another proposal has been made to allow the bucket elevator to escape in correspondence with variations in load by incorporating a load cell, a limit switch, etc. in an elevator post of the bucket elevator (see the description of the prior art in Japanese Patent Unexamined Publication No. 59-212325). In either of these proposals, however, there are cases where the timing of preventing the collision between the bucket and the bottom is delayed, so that it has not yet been possible to prevent the above-described accidents.
Furthermore, among bucket elevator-type continuous unloaders of this type, there is known one which makes it possible to selectively adopt two different scraping systems: a usual one (a lateral scraping system) in which the bulk cargo is scraped while the overall bucket elevator is being moved toward the side of each bucket, and another one (a catenary scraping system) which, when cleaning the bulk cargo from the bottom, makes it possible to absorb an impact caused by the vertical movement of the hull by setting the chains in a catenary (loose) state and to clean up the bulk cargo in the bottom (for instance, see Japanese Patent Unexamined Publication No. 59-212325). However, such a known unloader is arranged such that the bucket chains in a scraping section is set in a catenary state by adopting a system whereby lower sprockets of the bucket elevator are merely lifted by a cylinder.
However, (1) in recent years, holds of bulk cargo vessels have tended to become large in capacity as a result of the tendency of vessels to become large. Consequently, the length of a horizontal scraping portion in the unloader has also become large. (2). Therefore, all the horizontal forces for scraping (not only the force for tracting the bucket but also those acting vertically and horizontally) act on the chain. Hence, a need to strengthen the chain has arisen. (3). This has resulted in a heavier weight of the unloader. Consequently, in order to prevent the vertical or horizontal forces from being applied to the chain, it is necessary to install a guide rail in the horizontal scraping section and to provide each bucket with rollers guided by this guide rail.
Namely, a known unloader has the above-described drawbacks and needs, and if a guide rail and rollers are to be installed to satisfy these needs, there is a disadvantage in that catenary scraping cannot be effected.